As posted by others, Avira, Avast, AVG and Microsoft Security Essentials are very good. Also Vipre. If you use Microsoft Security Essentials, though, I recommend not using any of those other programs, except perhaps Avira. I have used MSE with Avira without any conflict, but the others mentioned will automatically disable both MSE and Windows Defender. Of course, if you use MSE, first disable Windows Defender.
Although Microsoft does not recommend using more than one antivirus program together, I have successfully used Avira Antivir Personal and Kingsoft Cloud Antivirus together, and also the combination MSE and Panda Cloud Antivirus. I configured each of them not to scan each other and my firewall to add both to my white list. I currently have both Kingsoft Cloud and Avira Antivir Personal on my XP without any problems. I also have Spyshelter (a free antikeylogger for XP) and Malware Fighter, without any conflicts. However, to make that combination a success on my XP, I uninstalled MSE completely, and disabled Windows Defender.